A conventional television system transmits a video signal containing a series of vertical synchronization (synch) pulses which occur approximately every 1/60th of a second (1/50th of a second in Europe). The vertical synch pulses provide timing information for the vertical sweep or deflection signal used to scan a cathode ray tube (CRT) to reconstruct the complete video image. Standard television receivers or video monitors contain circuitry which accepts each vertical synch pulse only after a vertical sweep is nearly complete. If a synch signal occurs before a prescribed time interval near the end of each vertical sweep, the circuitry is designed to treat the signal as a noise pulse and thereby blank out the perceived deleterious signal. In addition, conventional receivers or monitors contain a free-running vertical sweep generator which attempts to continue scanning the CRT by resetting the vertical sweep if a vertical synch pulse has not occurred within a given time period.
As a result of the so-called noise blanking and the free-running capability, if a vertical synch pulse is missed, or the frequency or phase of the synch pulses changes, the vertical sweep circuit responsive to the synch pulses will come "out-of-lock" with the vertical synch pulses. A time duration lasting through many vertical synch pulse intervals is often required for the vertical sweep circuit to re-lock onto the incoming vertical synch pulses. In addition, a conventional alternating current (AC) coupled sweep amplifier that drives the CRT is upset by the non-repetitive sweep input and hence rings and bounces for many vertical fields. During this transient, a blank bar is produced across the display of the television receiver or monitor, and the location of the image being displayed on the receiver or monitor bounces and rolls across the screen.
In video surveillance situations, it is oftentimes desirable to monitor a number of remote locations, such as entrances/exits of a building or stations along a production line, from a centralized monitoring location. For these situations, separate video cameras are stationed at each respective location to produce a view of a corresponding monitored location. If the view on each camera changes slowly, it is possible to use a single monitor to display on a time-shared basis the images produced by all the cameras, i.e., switch or sequence from one camera to the next every few seconds. To reduce system complexity and thereby mitigate operational expenses, the cameras often operate asynchronously. The rolling and bouncing image described above will likely occur when multiple, asynchronously operating cameras are used to provide separate video displays sequenced to a common monitor or group of monitors. In this instance, the composite synch signal which serves as the synch input to the monitor is composed of a series of synch pulses pseudo-randomly selected from the synch signals of the various cameras. The rolling and bouncing occurs because the periodicity of the standard vertical synch pulse is obliterated by switching back-and-forth among cameras that are not synchronized.
In the prior art of vertical deflection circuitry, U.S. Pat. No. 3,899,635 (issued to Steckler et al on Aug. 22, 1975) is representative of circuitry utilized to filter out impulse noise which could otherwise appear in the vertical synch signal and cause rolling of a displayed video image. As described in this patent, the vertical synch circuitry utilizes a resettable counter and searches for an externally applied synch pulse that occurs within a predetermined interval. If the pulse is not found, then the circuitry internally generates a vertical synch pulse.
Another type of vertical synch system is disclosed in U.S. Pat. No. 4,228,461 (issued to Weissmueller on Oct. 14, 1980). This system relies on searching for a vertical synch pulse within a pre-defined, relatively wide detection window. If such a pulse is found, it is used to initiate a vertical scan. If the pulse coincides with an internal control signal generated at a standard television (TV) frame rate, then the system switches to a relatively narrow detection window. Synch pulses that are received during the narrow window are also used to initiate vertical scans. In the event the pulses cease to coincide with the control signal for a given time interval, then the system reverts back to use of the relatively wide detection window.
Other previously devised solutions to this problem have been to use a very expensive digital time base corrector such as found in network TV or to arrange for all of the cameras in the system to be synchronized.
The prior art does not teach or suggest sweep generation techniques or deflection circuitry that can handle numerous unsynchronized incoming video signals which are purposely switched one-to-another for display on a single monitor. In fact, the synch disturbances in the single monitor case are seen as noise which is either rejected or has a long recovery time to resynchronize. Hence, a particular need exists in the art for a vertical deflection system, especially suited for use with a multiplicity of un-phased cameras utilizing a single display monitor, that, once an out-of-lock condition occurs, can re-establish a vertical lock condition on the next incoming vertical synch pulse in a vertical synch pulse stream derived from the individual synch streams of the multiple cameras.